linux-kernel - Re: [PATCH] applicom: Prevent unsigned wrap in ac

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Message-ID: <25e057c00908110154g48241b85td089e6726109b581@mail.gmail.com>
Date:	Tue, 11 Aug 2009 10:54:03 +0200
From:	roel kluin <roel.kluin@...il.com>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] applicom: Prevent unsigned wrap in ac_interrupt()

>> unsigned i wraps if this occurs in the first iteration.
>
> Could you elaborate? I don't quite understand the point.

`i' is unsigned. The last test in the loop is:

if(readb(apbs[i].RamIO + RAM_IT_TO_PC)) {
        /* There's another int waiting on this card */
        spin_unlock(&apbs[i].mutex);
        i--;
} else {
        spin_unlock(&apbs[i].mutex);
}

In the first iteration `i' is 0, so if this condition evaluates to true
then `i' becomes 0xffffffff (since it's unsigned), the for loop test
fails and the i++ never occurs.

>> Or do we know this can't happen?
>
> You mean the i--? It's followed by i++ in the for loop 3rd expression. Or?

No, I meant: do we know the test can't evaluate to true in the first iteration?

Thanks,

Roel
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